The tools used for working rocks in the field of mine working generally consist of a body in treated steel, the rear part of which constitutes a holder permitting the mounting of the tool in a support of a working machine and the front part of which carries a point of a hard material, such as tungsten carbide. The assembly of the tool is symmetrical in revolution and the holder is mounted in the tool support so that the tool may rotate about its axis during the operation of working the rock.
In order to increase the lifespan of the tool, it has been proposed to use, for constituting the point of the tool, a material which is more resistant to abrasion than tungsten carbide. For example, a tool has been proposed in Patent GB-A-2,146,058 which comprises an active part in a material which is resistant to abrasion, such as polycrystal diamond. To this end, a housing is provided in the end of the working part of the tool, made of tungsten carbide, and a composite abrasive component having an active end part made of polycrystal diamond is fixed inside the housing.
The front part of the tool constitutes a working surface with an overall frustoconical shape, having a component-holder part at its end with respect to which the active part of the composite abrasive component, made of polycrystal diamond, projects.
The advantage of this tool over tools comprising a tungsten carbide point is, however, limited. In fact, the diamond is approximately ten times as expensive as tungsten carbide and it would be necessary, for the price of the tool to remain competitive, for the lifespan of this diamond-charged tool to be increased to the same extent or to a similar extent.
In fact, it has been possible to observe that, in reality, the lifespan of such a tool comprising a diamond-charged abrasive component was considerably reduced because of the poor mechanical strength of the tungsten carbide in the component-holder part of the tool.
In fact, the presence of the housing in the component-holder part of frustoconical shape means that there is consequently an annular zone of small thickness made of tungsten carbide around the abrasive component.
As the tungsten carbide has poor bending and shear strength, cracks appear, during operation of the tool, in the component-holder part, constituting a weakened zone. The composite abrasive component can then be ejected from its housing after a relatively short time of use of the tool, which limits the overall lifespan of the tool and involves an increase in the operating coats of the working machine.
Depending on the nature of the rock being worked, an angle is provided at the vertex of the working frusto-conical part of the tool and, in particular, of the component-holder part which is adapted to the specific working conditions of the rock.
It is evident that the annular zone surrounding the composite abrasive component has a thickness which is greater, the greater the actual angle of the frustum of the cone of the component-holder part. An increase in this angle substantially in excess of the angle corresponding to the erosion profile of the working part of the tool leads, however, to an accelerated erosion of this working zone and of the component-holder part.